1. Field of the Invention
The present invention relates to a liquid crystal display apparatus using a two-terminal device as a switching device, and particularly relates to a reflection type liquid crystal display apparatus using a two-terminal switching device.
2. Description of the Related Art
Portable OA equipment such as personal computers have been developed in recent years. Accordingly, reduction of production cost and the reduction of the power consumption of liquid crystal display (LCD) devices used for such OA equipment have become important challenges.
In order to display high quality images by providing pixels at a high density, each pixel is supplied with a nonlinear active element (switching device) for driving the LCD apparatus. This system of driving is referred to as the "active matrix driving system". The switching devices are mainly available in two types: two-terminal devices such as MIM (metal insulator metal) elements, diodes, and varistors; and three-terminal devices such as TFTs (thin film transistors) and MOS-FETs (metal oxide semiconductor field effect transistors).
The three-terminal devices have excellent functions as switching devices and are suitable for displaying an image having various tones, for which different pixels are used for different tones. However, the three-terminal devices have inconveniences in that the complicated production process including repetition of exposure to light can easily cause defects in the obtained devices, resulting in low production yields.
The two-terminal devices, which have a simpler structure than that of the three-terminal devices, are produced by a simpler method due to, for example, the fewer steps of masking required. Accordingly, the production yield of the two-terminal devices is higher than that of the three-terminal devices and the production cost is lower than that of the three-terminal devices. For these reasons, methods for driving pixels using the two-terminal devices, especially by utilizing a nonlinear part of the operating characteristics of the two-terminal devices, have been actively studied and developed.
Among such two-terminal switching devices at present, the MIM-TFD (thin film diode) element using Ta.sub.2 O.sub.5 as the insulator layer is widely employed.
FIGS. 15 and 16A to 16C show an example of a conventional display apparatus 80 including a conventional MIM-TFD element. FIG. 15 is a plan view of a pixel and the vicinity thereof in the display apparatus 80. FIG. 16A to 16C are cross-sectional views of the display apparatus 80 shown in FIG. 15 looking along section lines 90-90', 91-91', and 92-92' in FIG. 15, respectively. Such a conventional structure of the MIM-TFD is described, for example, in "Liquid Crystal Display", Sharp Corp., Liquid Crystal Display Division, p. 71 and Japanese Patent Publication No. 1-35352.
The display apparatus 80 includes an insulating substrate 101 formed of glass or the like. A scanning line 102 formed of tantalum (Ta) is on a top surface of the substrate 101. A lower electrode 102' is branched from the scanning line 102 perpendicularly to the scanning line 102. A surface of the scanning line 102 and a surface of the electrode 102' are anodized to be an insulating layer 103. Specifically, the insulating layer 103 is formed of Ta.sub.2 O.sub.5. A rectangular metal layer 104 formed of Ta, Cr, or the like is on the substrate 101, covering the insulating layer 103. The metal layer 104 is arranged in a direction so as to cross the electrode 102'. A generally rectangular pixel electrode 105 as is shown in FIG. 11A is on the substrate 101, covering two ends of the metal layer 104. The pixel electrode 105 is transparent and conductive and formed of ITO (indium tin oxide) or the like.
The MIM element 108 includes a three-layer structure including the lower electrode 102', the insulating layer 103, and the metal layer 104. The electrode 102' acts as a first metal layer, the insulating layer 103 acts as an active layer, and the metal layer 104 acts as a second metal layer.
FIG. 17 shows a schematic view of the current-voltage (I-V) characteristic of the MIM element. The I-V characteristic of the MIM element is expressed by Equations (1), (2) and (3) by Poole-Frenkel current. ##EQU1## where q is the electric charge, n is the carrier density, .mu. is the mobility, .phi. is the depth of the trap, d is the thickness of the insulating layer, T is the temperature, K is the Boltzmann constant, and .di-elect cons. is the dielectric constant.
As is apparent from Equation (1), .beta., which is expressed by Equation (3) indicates the steepness of the I-V characteristic. It is preferable to obtain the highest possible value for .beta.. For example, the value of .beta. is approximately 3 to 4 inclusive in the MIM element including the insulating layer formed of Ta.sub.2 O.sub.5.
With regards to the reduction of power consumption, because a transmission type LCD which requires a back light does not take much advantage of the liquid crystal not emitting light by itself, a reflection type LCD which can display images without a back light has been proposed.
An LCD which utilizes a TN (Twisted Nematic) operation mode requires polarizers. When polarizers are used in the reflection type LCD, the efficiency of light utilization decreases due to the polarizer, resulting in a dark and indistinct display. Thus it is desired to use an LCD operation mode which does not use polarizers.
A typical LCD operation mode which requires no polarizer is the guest-host operation mode. The guest-host type LCD employs a liquid crystal material in which a dual-color dye which is anisotropic in absorbing visible light between the major axis and minor axis of the dye molecule is dissolved. An exemplary one of the guest-host modes is a cholesteric-nematic phase change guest-host (PCGH) mode, in which a p-type dye having a positive anisotropic dielectric constant (.DELTA..di-elect cons.=.di-elect cons..parallel.-.di-elect cons..perp.) is used as the dual-color dye. The PCGH mode, also called the White-Taylor mode after the names of the inventors, is capable of displaying white color nearly equal to paper white and has a feature capable of providing a high contrast.
An example of applying the PCGH mode to the LCD using a TFT element is described in SID 92 DIGEST p. 437, and an example of applying the PCGH mode to the LCD using an MIM element having the nonlinear resistive layer of Ta.sub.2 O.sub.5 is described in the Japanese Laid-open Patent Publication No. 5-12688.
A conventional reflection type LCD has several problems. The most serious problem among these is its low efficiency in utilizing light. This problem is caused by such factors as the requirement of the polarizers (which are indispensable when the TN mode is employed), a decrease in the aperture ratio due to the requirement of an area for bus lines, etc.
When the PCGH mode (described above) using no polarizer is employed to improve the light utilization efficiency, a bright reflection type LCD may be obtained. However, an experimental result has shown that there are some difficulties in driving an LCD in the PCGH mode by using an MIM element having a nonlinear resistive layer of Ta.sub.2 O.sub.5 as described in the Japanese Laid-open Patent Publication No. 5-12688. This is because the VAT (applied voltage-transmittance) characteristic of the liquid crystal used in the PCGH mode (called PCGH liquid crystal hereafter) is not very sharp.
In the PCGH mode, a focal conic state which is a transition state from a cholesteric state to a nematic state cannot be used for displaying, and the nonlinear characteristic of the MIM element having a Ta.sub.2 O.sub.5 layer is not so much sharp in the high electric field region, the ratio I.sub.20v /I.sub.5v which is an index of sharpness remains around 10.sup.3. The value of the ratio does not sufficiently satisfy the requirement of a very sharp I-V characteristic for the active element (the switching element such as MIM or TFT) in the PCGH mode described above.
The PCGH type LCD requires a switching element having a sufficiently sharper I-V characteristic. It is known that a very sharp I-V characteristic is an important desirable factor for preventing cross talk when driving a TN liquid crystal display apparatus.
On the other hand, with regards to the light utilization efficiency, the only way to increase the aperture ratio seems to be to use thinner bus lines as far as such a configuration of the pixel electrode and the switching element as adopted in the conventional LCD apparatus.
However, the thinner a bus line is, the higher the resistance of the bus line becomes. Thus improvement in the aperture ratio and the decrease in the resistance of bus line conflict with each other in the conventional LCD apparatus, and it has been impossible to achieve an increased aperture ratio and decreased resistance of the bus line at the same time. It has also been difficult to obtain a LCD having a large area.